One of the most efficient topologies in insulated AC/DC and DC/DC converters with low voltage output is the resonant insulated half-bridge (HB). The differences between a normal half-bridge stage and a resonant HB stage are well known in the literature. This also applies to the advantages associated with this latter solution, i.e. a higher efficiency and power density (due e.g. to zero-voltage-switching or ZVS operation of the switches, sinusoidal or quasi-sinusoidal waveforms and energy re-circulation after storing in the tank and releasing) and lower EMI (Electro Magnetic Interference) disturbance emissions.
A resonant insulated half-bridge may be made up of a HB insulated stage with the addition of a resonant tank, which leads to a configuration including:                a primary side half-bridge branch with two switches and decoupling capacitors;        an insulating transformer with e.g. a center-tap secondary winding; and        a secondary side rectifying and filtering stage.        
A resonant tank may be comprised of two or more inductive and/or capacitive components, connected in different combinations, leading to different kinds of parallel- or series-resonant structures. The presence of such a resonant structure leads to the gain of the converter being dependent on the switching frequency of the half-bridge. In the simplest embodiments, a resonant tank may include only two components (namely an inductor and a capacitor) and may give rise to either series- or parallel-resonance depending on the position of the insulating transformer with respect to the tank.
In operation, the switches may be driven by means of a square-wave with 50% duty-cycle and a dead-time to avoid cross-conduction in the switches and to reach ZVS condition. A Voltage Controlled Oscillator (VCO) may allow to control the output voltage of the converter by changing the switching frequency of the half-bridge.
Document EP-A-1 120 896 is generally representative of the prior art as considered herein.
A basic aim pursued in real applications may be to reduce the number and the dimensions of the components used and to increase inasmuch as possible the efficiency of the whole stage. Keeping EMI emissions as low as possible may be another desirable feature.
This may involve e.g. distributing the dissipated power homogenously over all the components and by integrating different functions in the same component.
In order to increase the efficiency of these typically low-voltage high-current converters, a current-doubler stage may be replaced for the output rectifying stage. This solution leads to a reduction in the current flowing in the secondary side of the transformer and to the elimination of the center tap in the transformer, which in turn leads to an increase in efficiency and a simplification and size decrease in the transformer, by the addition of a second output inductor.
A further attempt at reducing the number of the components may involve lowering the capacitive values of the HB capacitors, which permits to render them a part of the resonant tank, rather then using a third component. Another approach for decreasing the number of components involves using the parasitic parameters of the insulation transformer, such as the leakage or magnetizing inductance, as a part of the resonant tank, thus avoiding the use of an external inductor to provide series- or parallel-resonance.